Prop Inquiry.-

[Sea Rider]

Have in current use on a Tohatsu 2 strokes a 9 pitch factory prop which delivers at wot on a lightly loaded rib with solo driver 5600 rpm, I'm ordering a 8 pitch to be at/ around 5800 which is the max wot rpm for that engine. If order a 8 pitch 4 blade prop would my revs be pulled up same as a 3 blade 8 pitch would ? If both props at same max wot numbers, what would be the advantage to count with a 4 blade prop

Happy Boating

 

[steelespike]

First,what is the wot gps speed at 5600? It is usualy suggested to drop an inch going to a 4 blade from a 3.to maintain rpm.
Being a little motor it may gain some rpm with the 8" 4 blade but not like a 3 blade. The 4 blade may help get on plane easier.May allow a higher motor setup.
The 4 blade may have lower slip numbers.

 

[Sea Rider]

GPS WOT speed is 41,5 Km/h at 5600 rpm, I'm in pursuit of top wot rpm. not speed. If at 5800 my speed will be tad higher is welcome. If with a 3 blade 8 pitch will be spinning around 5800 rpm, assume will need to drop to a 7 pitch to maintain same rpm with a 4 blade alum prop, right ?

Happy Boating

 

[steelespike]

Is speed buy gps? prop calculator indicates your present slip is around 3% using 1.85 gear ratio and 5600 rpm. 3% is very low and unlikely.
Typical slip is usually in the low double digits.
The usual culprit when slip is really low is the tach. I used a chart to figure 41.5 Kmh to mph of 27.5.
Point is you may already be around 6000 rpm.
Do you know your ratio?

 

[Sea Rider]

Max speed was measured with 2 Garmin GPS with solo driver and very lightly loaded Rib on flat no wind water cond with engine at the sweet transom height on a 1 Km ride was : 41.5 Km/h. Max wot 5600 rpm was measured with 2 different induction tachs, that's Tiny and Hardline tachs, the difference between both was just 50 rpm, so very accurate. Gear ratio is 1.85

Just want to know if a 7 pitch alum 4 blade prop will bump my max wot numbers to or around 5800 rpm which should achieve if going by a 3 blade alum 8 pitch prop from standard factory delivered 9 pitch prop which produced just 5600 rpm.

Boating calculators theoretic numbers not always matches practice ones as hull design, water drag and other factors are not being taken into consideration. Everyone surprises down here the speed achieved with just a 18 HP on a 450 mt Rib that's rated for a minimum 30 HP engine.

Happy Boating

 

[steelespike]

On the 4 blade prop I really don't know I think smaller motors, under 25 hp, have a larger rpm change per inch.
You may note a 40 has more props available than your 18.
I may be whacky but I think a 1" drop will net more than a 200 rpm increase but a 4 blade may cost more rpm as well.
I'd be very interested to see the results. On the 4 blade I think 1" the rpm would be a wash.
A 7 " pitch is only one step from the smallest prop listed here. Seems extreme unless your hauling a lot of weight.
If your just shooting for an"ideal" rpm I think you are better off at 9" A 8" should give you brisker performance with a load
but at 5800 and 3% slip speed would be about 23 mph.Worth it for heavy loads but a lot of extra cycles running with a light load.

 

[Sea Rider]

New scenario :

New 8 prop pitch arrived, As with previous lightly loaded solo driving test, this time on flat calm open waters combo achieved 38.0 Km/h at 5800 revs, but at wot the engine revs high at 6200 rpm, 600 more than with previous test, so the statement that there's a 150-200 rpm increase going one less pitch does not apply on lighter boats.



5800 rpm which is the max wot rpm factory stated is achieved at slight passing 3/4 throttle grip. The hole shot at that rpm with 1-2-3 up is superb, much better than before. Although top speed at that rpm has decreased by 2 Km/h than with 9 pitch. Going from 5800 to 6200 the max achieved speed stays same as with 5800 rpm. If 130 kilos is added to deck weight the rpm will drop to 5800 rpm.

The nice issue is that hole shot has improved exponentially, can handle more load planing much faster. The only down issue that I see is that will need to throttle to 3/4 so not to over rev badly when solo driving. How much more rpm passing factory wot numbers would you need to pull a rod, a piston, break crank shaft ? If I knew there were going to be + 600 revs with a 8 prop pitch would have ordered a 8.5 prop pitch to gain just 300 + rpm. To be at the sweet max wot rpm could be a costly experiment under trial and error.

Happy Boating

 

[steelespike]

New scenario :

New 8 prop pitch arrived, As with previous lightly loaded solo driving test, this time on flat calm open waters combo achieved 38.0 Km/h at 5800 revs, but at wot the engine revs high at 6200 rpm, 600 more than with previous test, so the statement that there's a 150-200 rpm increase going one less pitch does not apply on lighter boats.



5800 rpm which is the max wot rpm factory stated is achieved at slight passing 3/4 throttle grip. The hole shot at that rpm with 1-2-3 up is superb, much better than before. Although top speed at that rpm has decreased by 2 Km/h than with 9 pitch. Going from 5800 to 6200 the max achieved speed stays same as with 5800 rpm. If 130 kilos is added to deck weight the rpm will drop to 5800 rpm.

The nice issue is that hole shot has improved exponentially, can handle more load planing much faster. The only down issue that I see is that will need to throttle to 3/4 so not to over rev badly when solo driving. How much more rpm passing factory wot numbers would you need to pull a rod, a piston, break crank shaft ? If I knew there were going to be + 600 revs with a 8 prop pitch would have ordered a 8.5 prop pitch to gain just 300 + rpm. To be at the sweet max wot rpm could be a costly experiment under trial and error.

Happy Boating

That 600 rpm gain is the difference I talked about in my post on the 24th.
. It has more to do with the relatively low hp than a light boat.
The weight is in proportion to that of a heavier boat and say a 40 or 50 hp.
Thats why a "smaller" motor has fewer prop sizes available. Each change affects a larger number.
I think you can see that a 1" pitch change will affect a 9.9 more than a 50.
I think you can see that if you go bak to the larger pitch you will have a bigger reduction in rpm than the rule of thumb.
You can see that the" lighter boat" doesn't enter into the prop change result.
A healthy motor can handle an occasional over rev easily but over rev sustained operation will result in premature failure.
A 2 stroke will handle over rev easier than a 4 stroke.
For instance if the valves in the 4 stroke start to "float" you could have a massive failure.
Thats why the 4 stroke has a over rev feature.
This is the second massive rpm change I've seen in a "smaller" motor.

 

[Sea Rider]

Not sure if its a low HP related theme, As an example, Tohatsu 18 and a 50 HP revs at same wot 5800 rpm. Factory delivered portable engines props are usually medium size pitch will work top on light to medium applications. So a 50 which is the max my 450 Rib accepts will probably over rev if solo driving loaded at 400 Kg with a 50.

Wasn't sure who posted achieving way more rpm that near rule of thumb 150-200 + revs with one pitch decrease. Even Iboats prop guidance page states same. Under which engine/boat condition would you say there's a 150-200 rpm increase ?

Anyway fearing that my boating Club's staff, girlfriend included goes full throttle due to throttle grip being extremely smooth tu turn fully inadvertently, will limit the internal throttlle mechanism to be at sweet max 5800 rpm with reduced throttle. At that rpm Rib handles very well and engine runs smooth.

Happy Boating

 

[steelespike]

Original wot rpm doesn't enter into the comparison.
If we add 200lbs to your 18 hp you will get a bigger drop in rpm
than adding 200lbs to to a 50 on your boat.
You spread additional weight through the rpm range the 18 will have fewer props to chose from
and they will make up more rpm.But will be less additional weight than the 50.

 

[Sea Rider]

Just following the tech conversation.

If adding 200 lbs of weight to my current combo the rpm drop will not be that significant as adding 286 lbs to pull my achieved wot 6200 rpm to safe 5800. All factory, Iboats included available prop selection goes from light, passing through middle to end at heavy loads applications. So is a relative small assorted variety to choose from. Besides if achieving + 500-600 rpm per pitch change, why would you need to have larger prop selection ?

I'll bet that if I ever sit a 50 HP on my 4.50 Mt Rib, that engine will over rev highly due to light boat overall weight and will need to go for one pitch more to be inside safe wot numbers. Ideal for my set up running solo or with reduced passengers while going wot cruising would be a 8.5 same diam prop pitch to be around 5900 max rpm which should be spot on for my boating purposes. Should achieve slight less best hole shot with slight better top speed than with an 8 prop pitch.

Meanwhile a 8.5 prop pitch is in order, have added a cheap plastic add on modification (X) that limits timing advance & carb sync travel to much less than when at wot as before.



Cam stops before roller starts going up cam's high angle, around 3/4 throttle. This setting dials safe 5900 max rpm at full grip.




For my peace of mind and investment, that's the only way for any savage riding my combo not to over rev it badly. 5900 rpm is best dialed for fast hole shot and slight less top speed than before, but quite satisfied with combo's general water performance.

Happy Boating